A strategy for increasing the energy release rate of aluminum by replacing the alumina passivation shell with aluminum iodate hexahydrate (AIH)

Abstract

Aluminum iodate hexahydrate (AIH) is an oxidizing salt that can be synthesized on the surface of nano-aluminum (nAl) particles, replacing the inherent alumina (Al2O3) passivation shell. For nAl particles encapsulated by AIH, rates of reaction have been shown to be on a time scale relevant to a detonation. Yet, little is known about the reaction mechanism between AIH and Al or how the Al2O3 passivation shell encapsulating nAl particles affects the reaction mechanism. This study explores the reactive nature of AIH and nAl by examining ignition and energy transfer of two different AIH + nAl formulations. The first is AIH synthesized on the surface of nAl particles to a concentration ratio of 80 wt% AIH to 20 wt% nAl. This composite particle lacks the Al2O3 shell that inherently passivates nAl and replacing it with AIH, these particles will be referred to as ParticleAIH+Al. In comparison, a discrete mixture of 80 wt% AIH powder combined with 20 wt% nAl powder is also examined and the mixture is referred to as MixAIH+Al. Laser ignition studies show ignition and burn times are reduced for ParticleAIH+Al compared to MixAIH+Al, owing to the presence of the Al2O3 diffusion barrier that inhibits ignition in the MixAIH+Al. Also, flame speed measurements reveal that ParticleAIH+Al propagate at 3062 m/s while the MixAIH+Al at 1366 m/s. Further analysis using thermal equilibrium calculations show hydrogen gas generation may facilitate heat transfer and energy propagation. Equilibrium analysis using differential scanning calorimetry also show the decomposition energy of pure AIH is nearly the same as the ignition energy of ParticleAIH+Al but twice that energy is required to ignite MixAIH+Al. All of these results provide a stronger foundation for the impedance of the Al2O3 passivation shell on the rate of energy release for Al reaction under equilibrium and non-equilibrium conditions.